Export wrapHtmlResponse helper method

[fabien]

When performing SSR using your own server (Express for example), it would be helpful to be able to hook into the existing functionality provided by the helpers at build-import-proxy.ts (more specifically: wrapHtmlResponse).

This is currently not possible, because the exports of the Snowpack package.json prevents low-level access. Therefor, it would be good to consider exporting them as part of the public JS api.

Also, the handleRequest method of SnowpackDevServer is not actually documented anywhere - however, it's extremely useful when using a custom server.